The invention herein described was made in the course of or under a contract thereunder with the United States Air Force Systems Command.
This invention relates to the purification of an olefinic endo-endo hexacyclic homodimer of norbornadiene or a mixture of isomers of said dimers of norbornadiene. The dimer or mixture of the dimers and its isomers hereinafter will be referred to as OHDNB. The invention also relates to the purification of the saturated endo-endo hexacyclic homodimer of norbornadiene or a mixture of the dimers and its isomers. The saturated dimer or a mixture of the dimers and its isomers hereinafter will be referred to as SHDNB.
Particularly the invention relates to removal of "color bodies" from OHDNB or SHDNB. "Color bodies" are color imparting substances which impart color to the liquid OHDNB or SHDNB. Often the color is a yellow ranging from a tint to a deep yellow. Some of the color bodies could be small amounts of organometallic catalyst used to form the dimer from the monomer. OHDNB and SHDNB without color bodies have the appearance of pure water.
Norbornadiene (bicyclo-(2.2.1)-2,5-heptadiene) can be prepared by reacting cyclopentadiene and acetylene at an elevated temperature, see U.S. Pat. No. 2,875,256 (Cl 260-666). Norbornadiene has the following structure: ##STR1## It can be dimerized into olefinic hexacyclic homodimers, one of which has the following structure: ##STR2## Dimerization of norbornadiene to compound I is disclosed in "The Stereochemical Course of Metal Catalyzed Cycloaddition Reactions of Norbornadiene", T. J. Katz et al, Tetrahedron Letters, No. 27, pp 2601-2605, 1967. The dimerization involves the use of a group VIII metal complex.
Among the compounds that can be formed from contacting compound I with hydrogen iodide is a compound having the following structure II. ##STR3## This is disclosed in "Dimerization and Trimerization of Norbornadiene by Soluble Rhodium Catalysts", Nancy Acton et al., Journal of the American Chemical Society, 94:15, July 26, 1972. Compound II, along with compound I is also disclosed in Chemical Abstracts, Vol. 70, 1969, page 265, 87128q disclosed in Journal of the American Society, 91:1, June 1, 1969, pages 206-8.
Hydrogenation of compounds I or II or an isomeric mixture containing them results in a compound or a mixture having a high density and a high heat of combustion making it useful as a missile fuel in either jet or rocket propulsion. The structure of the hydrogenated compound is as follows: ##STR4## Jet propulsion includes a jet engine which can be used for a missile, an aircraft and others and includes the three basic types, i.e., ramjet, turbojet and pulse jet. The term rocket generally refers to a device containing its own oxygen or oxidizing agent. An article in Aviation Week and Space Technology, Jan. 26, 1976, pages 111-113 discloses some of the high density hydrocarbon fuels that are under consideration as missile fuels.
However, the color bodies in OHDNB can diminish the activity of a hydrogenation catalyst used to convert the OHDNB to SHDNB. Furthermore the color bodies in the SHDNB can precipitate upon engine turbine blades and cause operational and maintenance problems. Thus it is advantageous to remove the color bodies from the aforementioned materials. Surprisingly, as discussed hereinafter, it has proven difficult to remove the color bodies.
The use of various clays as a decoloring agent in the treatment of hydrocarbons is known, see Encyclopedia of Chemical Technology, Kirk-Othmer, 2nd Edition, Vol 5, Clay (Survey). The foregoing reference classifies clays as to their geographic origin, chemical composition, and use. Serpentine mineral clays are classified as analogous to kaolin in which the latter's aluminum is replaced by magnesium. Thus a serpentine mineral clay can be considered as consisting of Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4, or a variation thereof, but as with all clays other minerals can be present. Synthetic clays such as one consisting of magnesium silicon, oxygen and hydrogen can also be prepared. Use of a magnesium silicate as a decolorization agent is disclosed in the following Chemical Abstracts, 1948, 2410 c; Vol 55, 23951 a; Vol 44, 4607 i; and Vol 86, 1977, page 372, 86; 22242 h. A bentonite clay is one rich in montmorillonite which contains both magnesium and aluminum and certain cations such as sodium as well as other elements. Generally, while bentonite differs from a montomorillonite clay in its geologic origin, either type can be used for decolorizing. Use of a bentonite or a montmorillonite for decoloring is disclosed in the following Chemical Abstracts: Vol 81, 1974, page 365, 103421q; Vol 79, 1973, page 97, 68067 p; Vol 73, 1970, page 86, 5221 q; Vol 68, 1968, page 5137, 52872 j; Vol 71, 1969, page 254, 64473 e; and Vol 70, 1969, page 72, 69460 n. A bentonite clay which has been washed with acid such as hydrochloric, sulfuric, phosphoric and the like is referred to as an acidic bentonite clay.